A seatback frame is the basic frame of a seatback that helps a passenger and a driver in comfortable and stable posture to thereby to allow them feel comfort and stable. Such a seatback frame is closely related to the safety of passengers, and thus there are a number of related regulations requiring no deformation or breakage against a certain level of force or impact. In order to meet these regulations and to ensure the safety of passengers, seat back frames have been made of steel. However, there is a problem of increasing cost and weight. Particularly, driving performance and fuel efficiency of vehicles are seriously lowered as the weight increases. In addition, when a seatback frame includes an armrest to provide more convenience for passengers and drivers, an additional manufacturing process for welding the seatback frame and the armrest frame after separately manufacturing them is required. As a result, it takes more time and cost, lowering productivity and economic efficiency. Therefore, a seat back frame has been manufactured using a single material such as a steel material, a glass mat thermoplastic (GMT) material or a long fiber reinforced thermoplastic resin. However, it is required to have a thick thickness because it does not have sufficient rigidity and strength against external impact, and thus the degree of design freedom was very low as it is manufactured by press-molding.